Recently, weight saving of tires has been pursued because of strong social demands for fuel efficiency of automobiles. Of tire members, weight saving has also been required for an inner liner serving as an air shutoff layer, which is disposed inside a tire and is required to reduce leakage of air from the inside to the outside of a pneumatic tire.
At present, for a rubber composition for the air shutoff layer, rubber formulation mainly composed of butyl rubber which contains, for example, 70 to 100% by mass of butyl rubber and 30 to 0% by mass of natural rubber is used to improve air permeation resistance of the tire. Further, the rubber formulation mainly composed of butyl rubber contains, in addition to butylene, about 1% by mass of isoprene, which allows intermolecular co-crosslinking with an adjacent rubber layer along with sulfur, a vulcanization accelerator, and zinc white. The butyl-based rubber having general formulation is required to have a thickness of about 0.6 to 1.0 mm for tires for passenger cars, and a thickness of about 1.0 to 2.0 mm for tires for trucks and buses. To pursue weight saving of tires, there has been a demand for a polymer which is more excellent in air permeation resistance and allows a further reduction in the thickness of the air shutoff layer, when compared with the butyl-based rubber.
In molding a green tire for a pneumatic tire, when an inner liner P is molded on a drum 5A as shown in FIG. 7, generally, an inner liner film P2 is bonded to an unvulcanized inner liner rubber P1 beforehand on a conveyer, with positions of both end edges in a longitudinal direction being aligned, to produce a laminate, the laminate is wound on a band over its entire circumference with inner liner film P2 of the laminate being disposed on an inner surface side, both end portions of the laminate are overlapped with each other at one location on the circumference to form a joint PJ, and thereafter a stitching roller is used to press joint PJ of the laminate and remove air.
In such a technique, since inner liner film P2 and unvulcanized inner liner rubber P1 are bonded beforehand with the positions of their both end edges in the longitudinal direction being aligned, and thereafter wound on the drum, joint PJ formed on the circumference of drum 5A inevitably has a large thickness when the both end portions of the laminate are overlapped and joined on the drum. Thus, even if the stitching roller is applied over joint PJ, air may remain at joint PJ, and if the remaining air expands by vulcanization molding of the green tire, joint PJ of laminate P may peel off.
In addition, since the end portions of laminate P form the joint at one location on the circumference of drum 5A in this technique, peeling-off of the joint of the inner liner of the molded green tire may cause a damage to an adjacent carcass ply.
It has been proposed in conventional techniques to use a thermoplastic elastomer for an inner liner with the intention to achieve weight saving of a pneumatic tire. However, the material, which is thinner and has a higher air permeation resistance than an inner liner made of butyl-based rubber, is inferior to the inner liner made of butyl-based rubber in vulcanization adhesive strength with insulation rubber and carcass ply rubber adjacent to the inner liner.
In particular, if the joint of the inner liner has a weak adhesive strength, the joint may peel off during driving, which may cause a reduction in the internal pressure of the tire, and burst of the tire. Further, since the joint has a structure in which another member is exposed inside, an air leakage path may be formed, and a reduction in the internal pressure of the tire is likely to be caused.
Patent Literature 1 (Japanese Patent Laying-Open No. 2009-208444) discloses a technique of molding an unvulcanized tire by bonding an inner liner film and an unvulcanized rubber sheet with both ends in an extending direction being displaced from each other, and winding the tacky body on a drum.
However, in order to displace the both ends in the extending direction from each other, it is necessary to cut each member one by one to have a constant size, and individually bond the members with being displaced from each other, which may deteriorate productivity. Further, depending on the bonding method, accuracy is deteriorated and air remains between the films, which may cause a damage during vulcanization of the tire.